The present invention relates to dielectric transducers and more specifically to dielectric transducers for use in testing or monitoring fluids.
It is well known that the dielectric properties of a sample may be useful in the assay of the sample. For example, the concentration of the constituents of a solution of two materials having different relative dielectric constants may be readily determined by measuring the dielectric constant of the solution. Typically, the dielectric constant of the sample is measured by capacitive methods, the sample providing all or part of the dielectric material separating the plates of a capacitor. From the geometry of the capacitor and its measured capacitance, the dielectric constant of the sample may be deduced. This value may then be compared with a calibration curve or table of values of the dielectric constant for varying known concentrations of the materials of interest. Typically, such values may be determined by directly substituting known concentrations of the materials in the capacitor.
To be directly comparable, the measurement of the sample must be made under identical conditions to those pertaining to the calibration. Thus, for instance, it is known that the dielectric constant of a material is temperature dependent. For example, the dielectric constants of most polar liquids vary inversely with the temperature. Because of such variations, all measurements of the sample should ideally be made at a pre-established temperature. Alternatively, the effects of the temperature dependence of the dielectric constant must otherwise be accounted for, as by measuring the temperature of the sample and applying an emperically determined correction factor to the capacictance measurement. While such approaches are consistent with standard laboratory practice, the requirement for temperature control or for a separate temperature measurement taken together with an independently determined thermal calibration curve detracts from the potential usefulness of such methods in many process control applications or in consumer goods.
Accordingly, it is a purpose of the present invention to provide a capacitive transducer suitable for the measurement of the dielectric properties of a sample and which incorporates a simple built-in reference standard, thereby insuring calibration of the transducer under varying conditions. It is a further object of the present invention to provide such a transducer which is temperature compensated.
These and other objects are met in the present invention of a capacitive transducer which comprises a pair of joined-together thin-film capacitors and a fluid-impervious container. The container may be filled with a standard solution of known composition. One of the two thin-film capacitors is immersed in the standard solution inside the container while the other is situated outside the container. The entire transducer may be immersed in a sample, and the capacitance of the outside capacitor, which is dependent on the dielectric constant of the sample, may be compared with that of the inside capacitor after the standard solution has reached thermal equilibrium with the sample.
It will be appreciated that the transducer so far described has the convenience of a self-contained reference standard as well as provision for thermal compensation. Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.